1.2. Basic Renal Process - Reabsorption

Question 1

What occurs in reabsorption?

Answer 1

Substances that the body wants are reabsorbed

Those it does not want will stay in the Tubule

Question 2

How much plasma per day is filtered through the Glomerular Capillaries into the Bowman’s Capsule?

Answer 2

180 litres per day

Question 3

How much urine do we excrete (on average) per day?

Answer 3

1-2 litres per day

Question 4

As 180 litres of plasma is filtered per day, and only 1-2 litres is excreted, what must happen?

Answer 4

Most of the plasma must be reabsorbed back into the peritubular capillaries

Question 5

What allows for a large amount of reabsorption in the peritubular capillaries?

Answer 5

1. Very low hydrostatic pressure

2. An increase in oncotic pressure

Question 6

What causes the very low hydrostatic pressure, favouring reabsorption?

Answer 6

There is little frictional resistance offered along the entire length of the peritubular capillary

Question 7

What causes the high oncotic pressure?

Answer 7

The plasma in the peritubular capillaries having a higher concentration of plasma proteins

Question 8

what are the methods by which substances are reabsrobed by the kidney?

Answer 8

1. Carrier mediated transport systems

2. Diffusion into the epithelial cells

Question 9

What are reabsorbed through carrier mediated transport mechanisms?

Answer 9

1. Glucose
2. Amino-acids
3. Organic-acids
4. Sulphate
5. Phosphate ions

Question 10

What do all of the carrier proteins have? And why does this occur?

Answer 10

A maximum transport capacity, due to the saturation of the carriers (Tm)

Question 11

What is the “Renal threshold”?

Answer 11

The plasma threshold at which saturation occurs

Question 12

If Tm is exceeded, that what happens to the substrates left in the tubule?

Answer 12

They are excreted in the plasma

Question 13

What is the renal plasma threshold for glucose?

Answer 13

10mmol/L - at a higher concentration that this it will appear in the urine (on urinalysis)

Question 14

How is the Tm of amino-acid reabsorption set in the kidneys?

Answer 14

Very high - so that urinary excretion does not occur

Question 15

What is amino-acid reabsorption regulated by?

Answer 15

Insulin and Counter-Regulatory hormones

Question 16

1. What substances does the kidney regulate by Tm mechanism?
2. How does this occur?

Answer 16

1. Sulphate and Phosphate ions
2. The Tm is set at a level whereby the normal plasma concentration causes saturation, and so any increase will be excreted

Question 17

What is reabsorbed by non-carrier mediated transport?

Answer 17

1. Sodium (Na)
2. Anions (e.g. Cl)
3. Water (H2O)
4. Other Solutes (e.g. K. Ca, Urea)

Question 18

What is the process of non-carrier mediated transport reabsorption?

Answer 18

1. Na pumps actively excrete Na from the Epithelial Cell into the Capillary Lumen
2. Na diffuses passively into the Epithelial Cells
3. This causes an electrochemical gradient, leading to anions to also be reabsorbed
4. Water moves by osmosis, following the solute reabsorption
5. The concentration of other solutes increases as fluid volume decreases, so permeable solutes are reabsorbed by diffusion

Question 19

What is non-actively reabsorbed solutes dependent on?

Answer 19

1. The amount of H2O removed (determining the concentration gradient)
2. The permeability of the membrane to any particular solute

Question 20

Why are all substances not reabsorbed then, due to the concentration gradient favouring it?

Answer 20

As not all substances can gain access through the tubule membrane

Question 21

What does glucose share its transporter with?

Answer 21

Sodium (Na)

Question 22

What is the process of glucose reabsorption?

Answer 22

1. Na moves down the electrochemical gradient (into the epithelial cell) using the SGLT protein, which pulls glucose into the cell (against its concentration gradient)
2. Glucose diffuses out of the basolateral side of the cell using the GLUT protein
3. Na is pumped out of the basolateral side of the cell by an NaKATPase transporter

Question 23

1. What causes a disruption in renal function?

2. Does this have effect on the Tm transport mechanisms?

Answer 23

1. Anything which will decrease active transport, e.g. a decrease in blood flow
2. Yes

Question 24

How does sodium concentration in the tubule effect the reabsorption of glucose?

Answer 24

1. High Na concentration facilitates it

2. Low Na concentration inhibits it

Question 25

Other than glucose, what else is the reabsorption of sodium (Na) related to?

Answer 25

Bicarbonate - for the acid base balance